In order to measure, with extreme precision, the position of machine components movable relative to each other, preferably optical position-measuring devices are used. They include one or more optical scanning units that are connected to a first machine component, as well as one or more measuring standards that are connected to a second machine component, the second machine component being movable relative to the first machine component. Displacement-dependent position signals, and consequently the relative position of the machine components, are able to be determined by the optical scanning of the measuring standard(s) with the aid of the scanning units. The measuring standards may be formed as one-dimensional scales or else as two-dimensional grid plates. In certain configurations, the scanning units of the position-measuring device are mounted on a movable machine component such as a table, for example, which must be positioned under a processing tool, the workpiece to be processed being disposed on the table. The intention is for the workpiece to be displaceable via the table in a displacement plane (XY plane) along two main axes of motion X, Y, while the other degrees of freedom (displacement along a Z-axis which is oriented perpendicularly to the XY plane, rotations about the X, Y and Z axes) are to be fixed or only slightly modulated. In a plane parallel to the displacement plane, one or more two-dimensional grid plates of the position-measuring device are disposed on the machine, against which the scanning units are able to measure. The grid plates are placed in stationary fashion about the specific processing tool.
The main objective of such a position-measuring device is to determine the position and location of the machine component at least with respect to the displacement degrees of freedom x, y along the main axes of motion X, Y, as well as with respect to the torsional degree of freedom RZ about the Z-axis. Hereinafter, the term 3-DOF measurement (DOF=Degree Of Freedom) is used in this regard. Moreover, for high-precision applications, it may be necessary to measure all six degrees of freedom of the specific machine component. They would also be the displacement degree of freedom z along the Z-axis, as well as the torsional degrees of freedom RY, RX about the Y-axis and X-axis, respectively. In this case, one refers to a 6-DOF measurement.
Such a system, which is used in the semiconductor industry for positioning a wafer (workpiece) under an exposure or inspection unit (processing tool), is described, for example, in U.S. Patent Application Publication No. 2007/0195296. The table T in a machine of this type is equipped with a position-measuring device having four combined scanning units E1 to E4 according to U.S. Pat. No. 7,573,581, as illustrated schematically in FIG. 1 of the present application. Each scanning unit E1 to E4 generates scanning signals or measured position values that contain two directional components of the scanning-unit position relative to the scanned grid plate:                Along a predefined direction in the plane of the grid plates; the measured position value in this direction is denoted hereinafter by Y(enc).        Along a distance perpendicular to the grid plate; this measured position value is denoted hereinafter by Z, the corresponding distance measurement as Z-measurement.        
Therefore, a scanning unit according to U.S. Pat. No. 7,573,581 may be viewed as a combination of a scanning unit having a measuring direction in displacement plane XY, and a scanning unit having a measuring direction Z perpendicular to displacement plane XY.
Scanning units E1 to E4 are mounted at the four corners of table T and, as illustrated in FIG. 1, are oriented at an angle relative to the two main axes of motion X, Y. Depending on the orientation, the scanning units therefore ascertain either measured position values Y(enc)=(X+Y)/√{square root over (2)} or measured position values Y(enc)=(X−Y)√{square root over (2)}. Moreover, each scanning unit E1 to E4 supplies information or measured position values with regard to its distance along the Z-axis to the grid plate.
In this system, scanning units E1 to E4 take measurements relative to four mutually adjoining grid plates M1 to M4 placed in a square, as illustrated in FIG. 2. Incidentally, the units of the axis legends in FIGS. 1 and 2 are selected arbitrarily. In the center of the grid-plate configuration, a larger area 3 is left open. The processing tool, e.g., the exposure or inspection unit, is located here. During machine operation, one of the four scanning units E1 to E4 may be located temporarily underneath cut-out area 3, and during this time, provide no measured values. Nevertheless, the machine position is able to be determined precisely in this case as well, since three scanning units engaged with grid plates M1 to M4 are sufficient for determining the six degrees of freedom of table T.
The precision of the particular machine is critically impaired, for example, by the following disturbance factors:                Distortions of grid plates M1 to M4 (static or changing slowly during machine operation, what is referred to as “drift,” often caused by temperature fluctuations)        Natural oscillations and vibrations of table T or of the machine component upon which the workpiece is located.        
The larger the workpiece, the more important the above-mentioned disturbance factors become, because for larger machine components and grid plates, it is more difficult to suppress vibrations and deformations by stiffening their construction. However, if measurement data concerning the scale deformations and the present state of excitation of the vibrations of the machine component is available, suitable measures may be taken for compensation or attenuation.
Such vibrations of the machine component represent oscillating deformations of the machine component, e.g., the table. They lead to a deflection of scanning units E1 to E4 disposed on it relative to their assumed position in the (hypothetically) rigid machine component. Consequently, the position of the machine component, which, without taking vibrations into account, is obtained directly from the measured values of scanning units E1 to E4, does not agree with the actual position of the machine component. Moreover, the position of the processing tool calculated from the machine-component position, thus, for example, the position of the exposure or inspection unit relative to the workpiece, i.e., the wafer, is not determined exactly, because the workpiece vibrates together with the machine component and deforms accordingly.
One possibility for detecting vibrations of machine components is based on the analysis of the history of the scanning-unit measured values over time or the corresponding analysis in the frequency space, and of the forces which were exerted on the machine component. In this regard, reference is made, for example, to U.S. Patent Application Publication No. 2011/0317142. The information thus obtained about the deformation of the machine component caused by a vibrational mode can be fed into the control of the actuators such that the error in the position of the processing tool relative to the workpiece caused by vibrations is sharply reduced.
However, such a vibration detection and compensation based exclusively on an analysis of the scanning-unit measured values over time has a number of disadvantages.
For example, the vibrations of the machine component are not detected via a measuring system during machine operation, but rather are estimated via a dynamic physical model that describes the effect of external forces on the machine component. The physical model may be ascertained from theoretical calculations or mechanical measurements. Small deviations of this model from the real mechanical behavior, as well as force attacks on the machine component not detected therefore contribute directly to a positioning error of the processing tool relative to the workpiece. The deviations of the model predictions from the real behavior cannot be detected during machine operation.
Moreover, the splitting of the measured values (and forces), described in U.S. Patent Application Publication No. 2011/0317142, into a position signal and a vibration signal is based on filters which process the variation in these quantities with time. For example, the filters are applied in the frequency space as bandpass or notch filters. In order to carry out such a frequency filtering, the time characteristic of the signal must be known over a time span on the order of one period of the vibration oscillation. Consequently, signal processing of this kind leads inevitably to a certain lag or to a latency of the control system, at least within the frequency bands which are assigned to the vibrations. Therefore, especially in the case of rapid acceleration processes or when attempting to actively dampen oscillations within a brief time, it must be expected that a sluggish control system is a disadvantage.